Not Applicable
1. Technical Field
This invention relates to uniformly distributing collimated light for information display, illumination, and direct lighting applications.
2. Background Art
It has long been a goal to develop large flat displays such as large screen television sets for consumer use. The expected performance for such displays is that they meet or exceed the performance of the well-known cathode ray tube (CRT) television (TV) display. This expected performance can include, for example, a wide viewing anglexe2x80x94with the present day CRT technology, a viewer can sit almost anywhere relative to the TV screen and experience essentially the same picture quality. This CRT quality level is typically inherent in both projection and emissive, such as plasma display, technologies.
Wall-mounted TV sets with large area plasma displays have been introduced into the consumer market. These plasma displays are relatively thin, typically less than five (5) inches (12.7 cm) thick, and have CRT-like viewing angles. However, large are plasma displays are currently too expensive for large-scale consumer use and tend to be of lower resolution than competing display technologies.
Projection displays require an unimpeded path between the projector and a screen. Backscreen projection displays typically include a large enclosure containing the projector, relay optics and a translucent screen. Because of these limitations, both types of projection displays are considered undesirable for large-scale consumer use.
Direct view liquid crystal displays, which are reasonably inexpensive to manufacture, are beginning to be introduced into the consumer TV set market. However, a typical liquid crystal display (LCD), such as a twisted-nematic LCD (TN-LCD) found in a typical laptop computer cannot normally provide the wide viewing angle expected by the consumer for a TV set. Several methods have been employed to widen the viewing angle of an LCD display and to also limit the overall thickness of an enclosure so that such a display could be used as a wall-mounted TV.
A typical approach to lighting an LCD display is to position a solid edge-illuminated light guide behind the LCD display. Collimation is known to improve the viewing angle of an LCD display when used in combination with a viewing screen applied to the output polarizer of the LCD display. It is also important to minimize surface reflections from the combination of viewing screen and LCD in order to provide sufficient readability in ambient lighting conditions. Previous attempts to minimize such reflections include the use of neutral density filters, triple notch filters, and circular polarizers.
There are several disadvantages associated with the use of solid edge-illuminated light guides as known in the art including the weight of such light guides and a decrease in light transmittance due to bulk material effects such as absorption and haze. For example, a solid glass light guide that would provide a 40-inch (101.6 cm) diagonal display the same luminance that a 10.4-inch (26.4 cm) diagonal display light guide would receive from a 6-millimeter thick solid glass light guide weighing 0.5 lb. (0.23 kg), would be 23-millimeter thick and weigh approximately 30 lb. (13.6 kg). Some optical glass exhibits light absorption losses as low as 0.002%/cm, while many optical plastics are closer to 0.10%/cm. It is worthy to note that this light absorption loss is not constant with wavelength, and therefore there is a corresponding change in the spectrum of the output light for a solid light guide device.
Previous attempts at both hollow and semi-hollow edge-illuminated collimated light guides have suffered from a non-uniform light output. Attempts to correct this non-uniformity including curving the extraction surface, employing very long extraction lengths, and tilting the centerline of collimated light source away from a light guide exit surface have met with varying degrees of success.
There is a need for an efficient and compact hollow edge-illuminated light guide for the distribution of collimated light to a liquid crystal display (LCD) that provides a uniform light output.
My invention comprises a method and apparatus for distributing collimated light and may be employed in lighting applications to enhance viewing angles of a wide angle Liquid Crystal Display (LCD). It can also be employed to distribute collimated light for other direct-lighting applications. More specifically, my invention is a light guide comprising a unique configuration of a hollow cavity surrounded by various optical films that receives collimated light through an entrance aperture and transmits the light uniformly from an exit aperture that is orthogonal to the entrance aperture. Advantageously the input light is both collimated and homogenized.
A significant advantage to the inventive hollow light guide is that any size guide can be assembled using the same microstructured parts and specular reflectors, so long as the collimation elements, such as morphing elements are the same.